1.3- Diketones dianions

Cydopentane reagents used in synthesis are usually derived from cyclopentanone (R.A. Ellison, 1973). Classically they are made by base-catalyzed intramolecular aldol or ester condensations (see also p. 55). An important example is 2-methylcydopentane-l,3-dione. It is synthesized by intramolecular acylation of diethyl propionylsucdnate dianion followed by saponification and decarboxylation. This cyclization only worked with potassium t-butoxide in boiling xylene (R. Bucourt, 1965). Faster routes to this diketone start with succinic acid or its anhydride. A Friedel-Crafts acylation with 2-acetoxy-2-butene in nitrobenzene or with pro-pionyl chloride in nitromethane leads to acylated adducts, which are deacylated in aqueous acids (V.J. Grenda, 1967 L.E. Schick, 1969). A new promising route to substituted cyclopent-2-enones makes use of intermediate 5-nitro-l,3-diones (D. Seebach, 1977). 

For the mechanistic course of the reaction the diketone 5 is assumed to be an intermediate, since small amounts of 5 can sometimes be isolated as a minor product. It is likely that the sodium initially reacts with the ester 1 to give the radical anion species 3, which can dimerize to the dianion 4. By release of two alkoxides R 0 the diketone 5 is formed. Further reaction with sodium leads to the dianion 6, which yields the a-hydroxy ketone 2 upon aqueous workup ... 

A key step in the approach to 3(2//)-furanone ring systems via the acid-catalyzed cyclization-dehydration of appropriately substituted a -hydroxy-l,3-diketones involves the acylation of a-hydroxy-ketone dianions 11141... 

Langer and coworkers have utilized dianions of 3-ketoesters or 1,3-diketones 2-220 with epibromohydrin (2-222) in SN/SN domino processes to give functionalized 2-alkylidene-5-hydroxymethyltetrahydrofurans 2-222 (Scheme 2.52) [127]. 

The corresponding endoperoxides of furans usually ring-open to afford 1,4-diketones. Such a sequence, followed by an intramolecular Diels Alder reaction was utilized in the synthesis of the tts-indacenyl dianion (6.7) 613>. 

This procedure represents a novel, convenient, and fairly general method for preparing y-aryl-/3-diketones. By this method the submitters have phenylated the dianion of 1-phenyl-1,3-butanedione (61%), 2,4-heptanedione (98%), 2,4-nonanedione (78%), 2,4-tridecanedione (53%), and 3,5-heptanedione (50%).6 Substituted diaryliodonium salts have also been used to produce l-(4-chlorophenyl)-2,4-pentadione (44%), 4-(4-methyl-phenyl)-l-phenyl-l,3-butanedione (44%), and l-(4-methyl-phenyl)-2,4-nonanedione (21%).6 Under these conditions no more than a trace, if any, of arylation at the a-position of the /3-diketones was observed by gas chromatography analysis. 

Addition of a 2-alkynoic acid to alkali amide in liquid ammonia initially gives a solution of the alkali salt of the carboxylic acid. If an excess of alkali amide is present, the weakly basic salt is further deprotonated at a position next to the triple bond 183], This double deprotonation which may be compared with the formation of di-anions from 1,3-diketones and alkali amides [71], is essentially complete. The high kinetic stability of the alkynoic acid-dianion may be explained on the basis of resonance stabilization ... 

The acyloin condensation is closely related to the radical anion coupling forming pinacolate anions two ester radical anions couple to form a dianion, which readily loses two alkoxide ions. The resulting diketone then is reduced by sodium, first to a semidione radical anion, then to the dianion. Finally, aqueous work-up produces the acyloin. Acyloins are convenient precursors for the generation of semidione radical anions. ... 

Aromatic esters may be used to aroylate the dianions derived from 1,3-diketones by reaction with potassium amide (60JOC538). Not only are acetyl and benzoyl acetones suitable for reaction, but alicyclic diketones and 2-hydroxyacetophenone are also acceptable. Cycliz-ation of the triketones occurs in cold sulfuric acid, presumably via the enolic form and the hemiacetal (Scheme 132). 

Ethyl -hydroxy benzoates.1 The dianions (LDA) of 1,3-diketones react with 1 to give ethyl 5,7-diketo-3-octenoates in 40-55% yield by an addition-elimination process. The products on reaction with dilute sodium ethoxide undergo dehydro-cyclization to afford ethyl p-hydroxybenzoates in 60-80% yield. 

Two main synthetic routes are available for the preparation of dithiolenes in the first and most frequently used method, either the free ethylenedithiol or an appropriate salt of the ethylene-dithiolato ligand dianion is reacted with a metal salt to produce anions of the dithiolenes, which may or may not be subsequently oxidized to the neutral species the second one, applied so far only to transition metal dithiolenes, converts vicinal diketones into dithiodiketones and reacts these either with zerovalent metals to form dithiolenes directly or uses metal salts to arrive at cationic species, which are reduced to the neutral dithiolenes either during the reaction or in a subsequent step. 

On the pages which follow, general methods are illustrated for the synthesis of a wide variety of classes of organic compounds including acyl isocyanates (from amides and oxalyl chloride p. 16), epoxides (from reductive coupling of aromatic aldehydes by hexamethylphosphorous triamide p. 31), a-fluoro acids (from 1-alkenes p. 37), 0-lactams (from olefins and chlorosulfonyl isocyanate p. 51), 1 y3,5-triketones (from dianions of 1,3-diketones and esters p. 57), sulfinate esters (from disulfides, alcohols, and lead tetraacetate p. 62), carboxylic acids (from carbonylation of alcohols or olefins via carbonium-ion intermediates p. 72), sulfoxides (from sulfides and sodium periodate p. 78), carbazoles... 

The dianions of 1,3-diketones react with (3, 3-difluorostyrenes to afford pyran 4-ones in moderate yield (Scheme 184) <1997H(45)37>. 

Phenylation and arylation have also been performed in bifunctional compounds such as malonates, /1-diketones and /1-ketoesters (Table 8.4). Diethyl malonate was arylated at room temperature affording mixtures of mono- and bis-arylated products, whereas isopropylidene malonate (Meldrum s acid) underwent bis arylation directly. Dimedone, the all carbon analogue of Meldrum s acid, was also mono-and bis-phenylated, with some concomitant O-phenylation [33], Generally, / -diketones show often ambident reactivity but the O-arylated product is normally the minor one an exception was noted in the triketone (PhCO)2CHCOPh which underwent mainly O-phenylation (68%) [33], Several dianions from /1-diketones have been arylated in high yields at the a-position, in a procedure superior to other methods [35]. 

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